Gruul Ragebeast

• WOTC Official Reminder Textofficial rules
  Fight — Each of those creatures deals damage equal to its power to the other.

• WWizards of the Coast2013-01-24
  Official Ruling
  Gruul Ragebeast's triggered ability isn't optional. However, if no opponent controls a creature when a creature enters under your control, the ability will be removed from the stack.
• WWizards of the Coast2013-01-24
  Official Ruling
  If one or both of the creatures that are supposed to fight are no longer on the battlefield when the ability resolves, the fight won't happen. Neither creature will deal or be dealt damage.
• WWizards of the Coast2013-01-24
  Official Ruling
  If the target creature is an illegal target when the ability tries to resolve, it won't resolve and none of its effects will happen. Neither creature will deal or be dealt damage.

Rulings via Scryfall